Ongoing improvements in personal computers allow users to prepare documents of increasing complexity and variety. Printer manufacturers are increasingly challenged to develop printing systems capable of printing high-quality, high-resolution heterogeneous documents. As the term is used herein, heterogeneous documents are documents that include more than one of the following: images, graphics, and text. Ideally, heterogeneous documents printed by printers accurately reproduce what is composed on a high-resolution computer monitor.
As an example of one of the challenges facing the printing industry, computer monitors are based on a color regime of red, green and blue pixels (RGB), whereas color printers, namely, inkjet printers, are typically based on a color regime of cyan, magenta, yellow and black (CMYK). The RGB color components of computer monitors are combined together in an “additive” way by mixing red, green and blue light rays to form a plurality of different displayable colors. In contrast, the CMYK components of color inks are applied to media in different combinations in a “subtractive” way to form a plurality of printable colors consistent with CMYK ink cartridges typically contained within the inkjet printer. Transforming images from “RGB space” to “CMYK space” necessary for printing the image on an inkjet printer requires the use of color-rendering techniques.
One color-rendering technique is halfioning. A halfioning algorithm is a process of transforming a continuous-tone image into a binary image that gives the illusion of the original continuous-tone image. Use of a single halfioning algorithm, when rendering a document, works well when the content of the document to be printed is homogeneous. For example, if the content of the document to be printed consists of only text, or only graphics, etc., an algorithm could be selected that will optimally render the text or graphics to the printer. In contrast, when a document is heterogeneous, containing various mixtures of text, graphics, or images, prior art single halfioning processes do not work as well because a given halfioning algorithm is directed to optimal rendering of only a single type of image.
Prior-art image rendering systems have attempted to address this problem. For example, U.S. Pat. Nos. 5,327,265 and 5,272,549 attempted to address the aforementioned problem by using a method wherein both text and images are printed separately, but in the same document. These methods include combining inkjet and laser printer components in a printing system in which the printed information is divided into color and text. The color images are printed using the inkjet printer component and the text (or non-color material) is printed by the laser printer component. This approach has the disadvantage of requiring very complex printers in order to print heterogeneous documents.
U.S. Pat. Nos. 5,704,021 and 6,040,927 disclose methods of color inkjet printing for use in printing documents having different color object types. Based on the identified color object type, selected color rendering options are used by the color inkjet printer to produce the document. The prior-art systems in the '021 and '927 patents require categorizing areas of a heterogeneous document to be printed, based on the type of objects contained in regions of the document, and then rendering the objects accordingly.
It would be advantageous to have an image rendering system capable of (1) improving print quality of heterogeneous images, (2) increasing the, speed at which heterogeneous images are rendered to a printer and, (3) rendering high quality heterogeneous images to relatively inexpensive conventional printers such as inkiet printers without being limited by object type.